The present invention relates to a liquid crystal display device wherein liquid crystal molecules in a liquid crystal layer are structured to be largely twisted between a pair of substrates, particularly, relates to a liquid crystal device of STN (Super Twisted Nematic) type or OMI (Optical Mode Interference) type formed in a monochrome type based on the STN type liquid crystal device. Further, the present invention relates to ECB (Electrically Controlled Birefringence) type liquid crystal device wherein liquid crystal molecules are structured to be oriented homogenously between a pair of substrates.
The liquid crystal device of the present invention can be used widely in various displays.
In recent years, developments and researches for the liquid crystal display device have been remarkably proceeded, for example, the STN type liquid crystal display device has been developed in place of formerly general TN type (Twisted Nematic type). In the STN type liquid crystal device, liquid crystal molecules are structured to be twisted largely between a pair of substrates, and the orientation direction of liquid crystal molecules on the substrate and the transmission axis (absorption axis) of each polarizer are arranged at a certain angle to allow the liquid crystal to be provided with a birefringence effect.
In the STN type liquid crystal device as mentioned above, since the orientation direction of liquid crystal is changed nearby the threshold voltage and the displayed color changes by the birefringence effect in accordance with the drive voltage, a speedy luminance change by the voltage and an excellent time-division characteristic can be obtained.
However, on the other hand, since the STN type liquid crystal device uses the birefringence effect of the liquid crystal, the displayed color is changed in accordance with the product .DELTA.n.multidot.d of the refractive index anisotropy .DELTA.n and the thickness d of liquid crystal layer. The above .DELTA.n.multidot.d has an appearance value changeable in accordance with the visual angle direction, namely the direction of seeing the liquid crystal display device. Therefore, there is a problem that the displayed color looks changeable in accordance with the visual angle direction.
The first aspect of the present invention is to provide a STN type liquid crystal display device in which the display color change caused by the visual angle direction is reduced and the visual angle characteristic is improved.
Since the present STN type liquid crystal display device uses the birefringence effect of liquid crystal, the liquid crystal cell is colored, therefore, it is hard to obtain a color liquid crystal display device. Further, a color tone ununiformity in the liquid crystal cell is caused by the slight change of the liquid crystal cell thickness.
In view of the problem as described above, in recent years, the OMI type liquid crystal display device formed by rendering the STN type liquid crystal display device monochrome by adjusting the angle of a polarizer and the retardation value has been developed.
However, since such an OMI type liquid crystal display device uses the birefringence effect of the liquid crystal like the STN type liquid crystal device, the displayed color is changed in accordance with the product .DELTA.n.multidot.d of the refractive index anisotropy .DELTA.n and the thickness d of liquid crystal layer. The above .DELTA.n.multidot.d changes in an appearance value in accordance with the visual angle direction, namely the direction of seeing the liquid crystal display device. Therefore, there is a problem that the color looks changeable in accordance with the visual angle direction.
Therefore, the second aspect of the present invention is to provide OMI type liquid crystal display device in which the display color change caused by the visual angle direction is reduced and the visual angle characteristic is improved.
On the other hand, an ECB type liquid crystal display device utilizing the birefringence effect electrically controlled has a structure that a liquid crystal cell sandwiches therebetween the liquid crystal layer to perform the liquid crystal molecules to be oriented homogenously between the upper and lower substrates, and polarizers are arranged on both sides of the liquid crystal cell so that the transmission axis or absorption axis of the polarizer nearby the liquid crystal layer is shifted from the orientation direction of the liquid crystal molecules, thereby the liquid crystal cell can be colored by utilizing the birefringence property of liquid crystal molecules. And the displayed color is changed in accordance with the variable orientation state of the liquid crystal layer by applied voltage. For example, Japanese Patent Laid-Open SHO 62-275227, corresponding to U.S. Pat. No. 4,813,770, discloses the ECB type liquid crystal display device of the DAP system performing a light modulation by changing the birefringence degree of liquid crystal layer while applying the voltage to the vertically oriented liquid crystal layer.
However, the conventional ECB type liquid crystal display device has the problem that the displayed color is remarkably changed in accordance with the visual angle direction for the reason that the distance in liquid crystal layer where the light is actually passing through is changed by the visual angle and the degree of birefringence in appearance is changed in accordance with the relative angle formed between the orientation direction of liquid crystal molecules and the visual angle direction.
The third aspect of the present invention is to provide the ECB type liquid crystal display device which is formed without such a disadvantage as described above, and is remarkably less in change of displayed color dependent on the visual angle and excellent in display quality.
Namely, the present inventors have proceeded their research for finding a solution to the problem in the conventional liquid crystal display device, and found that the liquid crystal display device superior to the conventional each type of liquid crystal display device in the visual angle characteristic is obtained by utilizing the liquid crystal display device which comprises (1) a liquid crystal cell wherein the liquid crystal layer comprising a liquid crystal component having a positive dielectric anisotropy is sandwiched between a pair of substrates, (2) a pair of polarizers arranged to sandwich the liquid crystal layer therebetween and (3) a birefringence layer having refractive index anisotropy and transparency provided between the liquid crystal layer and the polarizer on at least one side of the liquid crystal layer, wherein the angle formed between the maximum refractive index direction of the birefringence layer face and the direction of transmission axis or absorption axis of the polarizer adjacent to the birefringence layer is not more than 5.degree..
Namely, the present inventors found that the ECB type liquid crystal display device which is excellent in visual angle characteristic can be obtained by structuring so that the liquid crystal molecules in the liquid crystal cell of the above liquid crystal display device are oriented homogeneously between the pair of substrates, and they have been succeeded the present invention based on this finding.
Further, the present inventors found that the excellent STN type liquid crystal display device can be obtained from the liquid crystal display device having (1) the above-mentioned conditions and (2) the construction that its liquid crystal molecules are remarkably twisted in the liquid crystal layer between a pair of substrates, wherein the orientation direction of liquid crystal molecules adjacent to the polarizer is shifted from the direction of a transmission axis or absorption axis of the polarizer, and the angle of the direction of optical axis in the birefringence layer projected to the birefringence layer face formed to the orientation direction of liquid crystal molecules adjacent to the birefringence layer is limited in certain positive or negative angle ranges. Further, they found that the excellent OMI type liquid crystal display device can be obtained by limiting (1) the product of refractive index anisotropy .DELTA.n of the liquid crystal component and the thickness d of liquid crystal layer, (2) the angle formed between the transmission axis of both polarizers and (3) the angle formed between the orientation direction of liquid crystal molecules in the center of the thickness of liquid crystal layer and the bisector between transmission axes of the pair of polarizers, respectively in certain ranges, and have been succeeded the present invention based on these findings.
Namely, the STN type, OMI type and ECB type liquid crystal display devices of the present invention have advantages that the color change in accordance with the visual angle change is decreased and that high contrast displayed images can be generated.